The icy tundra areas mostly don't have good soil. Even if they get the temperatures of the American midwest they will not produce nearly the same calories/year

The icy tundra areas mostly don't have good soil. Even if they get the temperatures of the American midwest they will not produce nearly the same calories/year/acre. So even if populations could all start migrating north (without somehow triggering world-spanning conflicts as millions of people get forced across national borders) their food source won't be coming with them.